The body's defense system against microbes as well as the body's defense against other chronic diseases such as those affecting cell proliferation is mediated by early reactions of the innate immune system and by later responses of the adaptive immune system. Innate immunity involves mechanisms that recognize structures which are for example characteristic of the microbial pathogens and that are not present on mammalian cells. Examples of such structures include bacterial liposaccharides, (LPS) viral double stranded DNA, and unmethylated CpG DNA nucleotides. The effector cells of the innate immune response system comprise neutrophils, macrophages, and natural killer cells (NK cells). In addition to innate immunity, vertebrates, including mammals, have evolved immunological defense systems that are stimulated by exposure to infectious agents and that increase in magnitude and effectiveness with each successive exposure to a particular antigen. Due to its capacity to adapt to a specific infection or antigenic insult, this immune defense mechanism has been described as adaptive immunity. There are two types of adaptive immune responses, called humoral immunity, involving antibodies produced by B lymphocytes, and cell-mediated immunity, mediated by T lymphocytes.
Two types of major T lymphocytes have been described, CD8+ cytotoxic lymphocytes (CTLs) and CD4 helper cells (Th cells). CD8+ T cells are effector cells that, via the T cell receptor (TCR), recognize foreign antigens presented by class I MHC molecules on, for instance, virally or bacterially infected cells. Upon recognition of foreign antigens, CD8+ cells undergo an activation, maturation and proliferation process. This differentiation process results in CTL clones which have the capacity of destroying the target cells displaying foreign antigens. T helper cells on the other hand are involved in both humoral and cell-mediated forms of effector immune responses. With respect to the humoral, or antibody immune response, antibodies are produced by B lymphocytes through interactions with Th cells. Specifically, extracellular antigens, such as circulating microbes, are taken up by specialized antigen-presenting cells (APCs), processed, and presented in association with class II major histocompatibility complex (MHC) molecules to CD4+ Th cells. These Th cells in turn activate B lymphocytes, resulting in antibody production. The cell-mediated, or cellular, immune response, in contrast, functions to neutralize microbes which inhabit intracellular locations, such as after successful infection of a target cell. Foreign antigens, such as for example, microbial antigens, are synthesized within infected cells and resented on the surfaces of such cells in association with Class I MHC molecules. Presentation of such epitopes leads to the above-described stimulation of CD8+ CTLs, a process which in turn also stimulated by CD4+ Th cells. Th cells are composed of at least two distinct subpopulations, termed Th1 and Th2 cells. The Th1 and Th2 subtypes represent polarized populations of Th cells which differentiate from common precursors after exposure to antigen.
Each T helper cell subtype secretes cytokines that promote distinct immunological effects that are opposed to one another and that cross-regulate each other's expansion and function. Th1 cells secrete high amounts of cytokines such as interferon (IFN) gamma, tumor necrosis factor-alpha (TNF-alpha), interleukin-2 (IL-2), and IL-12, and low amounts of IL-4. Th1 associated cytokines promote CD8+ cytotoxic T lymphocyte T lymphocyte (CTL) activity and are most frequently associated with cell-mediated immune responses against intracellular pathogens. In contrast, Th2 cells secrete high amounts of cytokines such as IL-4, IL-13, and IL-10, but low IFN-gamma, and promote antibody responses. Th2 responses are particularly relevant for humoral responses, such as protection from anthrax and for the elimination of helminthic infections.
Whether a resulting immune response is Th1 or Th2-driven largely depends on the pathogen involved and on factors in the cellular environment, such as cytokines. Failure to activate a T helper response, or the correct T helper subset, can result not only in the inability to mount a sufficient response to combat a particular pathogen, but also in the generation of poor immunity against reinfection. Many infectious agents are intracellular pathogens in which cell-mediated responses, as exemplified by Th1 immunity, would be expected to play an important role in protection and/or therapy. Moreover, for many of these infections it has been shown that the induction of inappropriate Th2 responses negatively affects disease outcome. Examples include M tuberculosis, S. mansoni, and also counterproductive Th2-like dominated immune responses. Lepromatous leprosy also appears to feature a prevalent, but inappropriate, Th2-like response. HIV infection represents another example. There, it has been suggested that a drop in the ratio of Th1-like cells to other Th cell populations can play a critical role in the progression toward disease symptoms.
As a protective measure against infectious agents, vaccination protocols for protection from some microbes have been developed. Vaccination protocols against infectious pathogens are often hampered by poor vaccine immunogenicity, an inappropriate type of response (antibody versus cell-mediated immunity), a lack of ability to elicit long-term immunological memory, and/or failure to generate immunity against different serotypes of a given pathogen. Current vaccination strategies target the elicitation of antibodies specific for a given serotype and for many common pathogens, for example, viral serotypes or pathogens. Efforts must be made on a recurring basis to monitor which serotypes are prevalent around the world. An example of this is the annual monitoring of emerging influenza A serotypes that are anticipated to be the major infectious strains.
To support vaccination protocols, adjuvants that would support the generation of immune responses against specific infectious diseases further have been developed. For example, aluminum salts have been used as a relatively safe and effective vaccine adjuvants to enhance antibody responses to certain pathogens. One of the disadvantages of such adjuvants is that they are relatively ineffective at stimulating a cell-mediated immune response and produce an immune response that is largely Th2 biased.
It is now widely recognized that the generation of protective immunity depends not only on exposure to antigen, but also the context in which the antigen is encountered. Numerous examples exist in which introduction of a novel antigen into a host in a non-inflammatory context generates immunological tolerance rather than long-term immunity whereas exposure to antigen in the presence of an inflammatory agent (adjuvant) induces immunity. (Mondino et al., Proc. Natl. Acad. Sci., USA 93:2245 (1996); Pulendran et al., J. Exp. Med. 188:2075 (1998); Jenkins et al., Immunity 1:443 (1994); and Kearney et al., Immunity 1:327 (1994)).
A naturally occurring molecule well known to regulate adaptive immunity is CD40. CD40 is a member of the TNF receptor superfamily and is essential for a spectrum of cell-mediated immune responses and required for the development of T cell dependent humoral immunity (Aruffo et al., Cell 72:291 (1993); Farrington et al., Proc Natl Acad Sci., USA 91:1099 (1994); Renshaw et al., J Exp Med 180:1889 (1994)). In its natural role, CD40-ligand expressed on CD4+ T cells interacts with CD40 expressed on DCs or B cells, promoting increased activation of the APC and, concomitantly, further activation of the T cell (Liu et al Semin Immunol 9:235 (1994); Bishop et al., Cytokine Growth Factor Rev 14:297 (2003)). For DCs, CD40 ligation classically leads to a response similar to stimulation through TLRs such as activation marker upregulation and inflammatory cytokine production (Quezada et al. Annu Rev Immunol 22:307 (2004); O'Sullivan B and Thomas R Crit Rev Immunol 22:83 (2003)) Its importance in CD8 responses was demonstrated by studies showing that stimulation of APCs through CD40 rescued CD4-dependent CD8+ T cell responses in the absence of CD4 cells (Lefrancois et al., J Immunol. 164:725 (2000); Bennett et al., Nature 393:478 (1998); Ridge et al., Nature 393:474 (1998); Schoenberger et al., Nature 393:474 (1998). This finding sparked much speculation that CD40 agonists alone could potentially rescue failing CD8+ T cell responses in some disease settings.
Other studies, however, have demonstrated that CD40 stimulation alone insufficiently promotes long-term immunity. In some model systems, anti-CD40 treatment alone insufficiently promoted long-term immunity. Particularly, anti-CD40 treatment alone can result in ineffective inflammatory cytokine production, the deletion of antigen-specific T cells (Mauri et al. Nat Med 6:673 (2001); Kedl et al. Proc Natl Acad Sci., USA 98:10811 (2001)) and termination of B cell responses (Erickson et al., J Clin Invest 109:613 (2002)). Also, soluble trimerized CD40 ligand has been used in the clinic as an agonist for the CD40 pathway and what little has been reported is consistent with the conclusion that stimulation of CD40 alone fails to reconstitute all necessary signals for long term CD8+ T cell immunity (Vonderheide et al., J Clin Oncol 19:3280 (2001)).
Various agonistic antibodies have been reported by different groups. For example, one mAb CD40.4 (5c3) (PharMingen, San Diego Calif.) has been reported to increase the activation between CD40 and CD40L by approximately 30-40%. (Schlossman et al., Leukocyte Typing, 1995, 1:547-556). Also, Seattle Genetics in U.S. Pat. No. 6,843,989 allege to provide methods of treating cancer in humans using an agonistic anti-human CD40 antibody. Their antibody is purported to deliver a stimulatory signal, which enhances the interaction of CD40 and CD40L by at least 45% and enhances CD40L-mediated stimulation and to possess in vivo neoplastic activity. They derive this antibody from S2C6, an agonistic anti-human CD40 antibody previously shown to deliver strong growth-promoting signals to B lymphocytes. (Paulie et al., 1989, J. Immunol. 142:590-595).
Because of the role of CD40 in innate and adaptive immune responses, CD40 agonists including various CD40 agonistic antibodies have been explored for usage as vaccine adjuvants and in therapies wherein enhanced cellular immunity is desired. Recently, it was demonstrated by the inventor and others that immunization with antigen in combination with some TLR agonists and anti-CD40 treatment (combined TLR/CD40 agonist immunization) induces potent CD8+ T cell expansion, eliciting a response 10-20 fold higher than immunization with either agonist alone (Ahonen et al., J Exp Med 199:775 (2004)). This was the first demonstration that potent CD8+ T cell responses can be generated in the absence of infection with a viral or microbial agent. Antigen specific CD8+ T cells elicited by combined TLR/CD40 agonist immunization demonstrate lytic function, gamma interferon production, and enhanced secondary responses to antigenic challenge. Synergistic activity with anti-CD40 resulting in the induction of CD8+ T cell expansion has been shown with agonists of TLR1/6, 2/6, 3, 4, 5, 7 and 9.
To increase the effectiveness of an adaptive immune response, such as in a vaccination protocol or during a microbial infection, it is therefore important to develop novel, more effective, vaccine adjuvants. The present invention satisfies this need and provides other advantages as well.
Also, it is important to develop effective immune adjuvants which are effective at doses which do not elicit adverse side effects such as liver toxicity. Particularly it has been reported by Vanderheide et al., J Clin. Oncol. 25(7)876-8833 (March 2007) that a 0.3 mg/kg is the maximum tolerated dose for an exemplified agonistic antibody and that higher doses may elicit side effects including venous thromboembolism, grade 3 headache, cytokine release resulting in toxic effects such as chills and the like, and transient liver toxicity. Also, it has been reported by Vanderheide et al., J Clin. Oncol. 19(23):4351-3 (2001) that the maximum tolerated dose for a hCD40L polypeptide described therein was 0.1 mg/kg/day and that when the polypeptide was administered at higher doses of 0.15 mg/kg/day they observed liver toxicity characterized by grade 3 or 4 liver transaminase elevated levels in subjects treated.